1. Field of the Invention
The present invention relates to an inner-rotor motor. In particular, the present invention relates to an improvement in a motor in which a stator is arranged radially outside and opposite to a rotor magnet.
2. Description of the Related Art
In a common inner-rotor motor, a rotor magnet is fixed to a shaft, and a stator is arranged radially outside the rotor magnet. In addition, bearings arranged to support the shaft are arranged on an upper side and a lower side of the rotor magnet, and each of the bearings is supported by a bracket arranged to cover the stator. The shape of the stator depends on desired characteristics of the motor, such as power and torque. Therefore, different brackets should be used for different motors having different desired characteristics, and it is impossible to share the same bracket between the different motors to achieve a reduction in costs of the motors.
Meanwhile, JP-A 11-220860, for example, describes a motor in which both ends of a stator are held by two brackets, and these brackets are fixed and fastened by a fixing screw. Adoption of a structure described in JP-A 11-220860 makes it possible to use the same brackets for different motors whose stators have different axial dimensions.
However, the motor described in JP-A 11-220860 has a problem in that use of the fixing screw is necessary to fix and fasten the brackets, and the number of parts is increased. In addition, the motor described in JP-A 11-220860 has a problem in that fixing screws having different lengths should be used in accordance with the axial dimension of the stator. Furthermore, the motor described in JP-A 11-220860 has a problem in that a space in which the fixing screw is inserted should be secured in the stator, and this leads to a deterioration in magnetic properties of a stator core and a decrease in an energy efficiency of the motor.
An interspace between the brackets depends on the axial dimension of the stator, and is affected by an error in the shape of the stator. Therefore, in the case where ball bearings are used to support a shaft, and a preload in accordance with the interspace between the brackets is applied to each of the ball bearings, the error in the shape of the stator affects the preload applied to each of the ball bearings. That is, variations in manufactured stators may cause vibrations and noise while motors are running, and may cause deterioration in durability of the motors.